G

Word

Meaning

Galactic Centre

The nucleus, or centre, of a galaxy, usually densely populated by stars. The galactic centre of the Milky Way is about 28,000 light years from Earth.

Galatea

Moon of Neptune, fourth closest to the planet. Galatea is 175 kilometres (109 miles) in diameter and orbits Neptune at an average distance of 61,953 km (38,496 miles). It was discovered by Voyager 2 in 1989.

Galaxy

A large group of stars, bound together by gravity. Quite often, the galaxy will form a spiral or circular shape.

Galilean Satellites / Galilean Moons

Jupiter's four largest moons, discovered by Galileo in 1610. These are (in order of increasing distance from Jupiter) Io, Europa, Ganymede and Calisto. Ganymede is the largest moon in the Solar System.

Galle, Johann Gottfried

German astronomer who made the first observation of Neptune, assisted by Heinrich Louis d'Arret. This observation was based on predictions of the planet's position by French mathematician Le Verrier.

Gamma Rays

A form of energy such as light, but made from shorter and more powerful waves.

Ganymede

Moon of Jupiter, largest moon in the Solar System. Ganymede's surface is more heavily cratered on one side than the other. It is about 5,262 km in diameter (a greater diameter than both Mercury and Pluto) and takes 7 days and 3 hours to complete an orbit of Jupiter.

Gas Giant

A planet with a small, possibly rocky core, surrounded by a deep atmosphere. This atmosphere is made up mainly of hydrogen and helium gases. There are four Gas Giants in your Solar System. These are Jupiter, Saturn, Uranus and Neptune. The majority of planets discovered around other stars are also Gas Giants.

Geostationary / Geosynchronous Orbit

An orbit of a satellite around Earth which is in a stationary position above a set point on Earth, taking the same length of time to orbit Earth as it takes for Earth to spin on its axis (24 hours). Communications satellites (for example, those used to broadcast satellite television) and weather satellites are most commonly placed into geostationary orbits.

Gravity

A force which attracts objects of mass to objects of a greater mass. This force is what created the stars and planets, and keeps people stuck to Earth. The more massive an object, the stronger its gravitational force. The Sun's gravity is able to keep planets many millions of miles away from it in orbit around it.

Gravity Assist

A method used to propel a spacecraft to a planet or moon using the gravity of another planet or moon. This is done by the object entering into the orbit of a planet, therefore now travelling around the Sun at the same speed as the planet. It then gets slung out of this orbit and uses the velocity it has built up to travel faster to its next destination. The first mission to use this was Mariner 10 which used the gravity of Venus to take it to Mercury. The most famous mission to use gravity assist was the Voyager which used the gravity of the four Gas Giants to propel the craft to each planet and now on their ways out of the Solar System.